1. Field of the Invention
The present invention relates to a lens driving apparatus and method. More particularly, the present invention relates to a lens driving apparatus and method in which changes in a moving speed of a movable lens in a lens assembly are prevented to remove unfamiliar feeling in manipulation of a user.
2. Description Related to the Prior Art
A lens driving apparatus (lens assembly) is provided in a digital camera, video camera or the like for focusing, zooming and the like. An example of a motor in the lens driving apparatus is a voice coil motor, which includes a magnet, a yoke and a coil, and can operate at a high speed. In the voice coil motor, a current flows through the coil, which is driven by electro-magnetic force and moves along the yoke. A lens (lens optics) is coupled with the coil, and can be moved together with the coil. An example of the lens is a focus lens, zooming lens (variator lens) and the like. The focus lens is movable to adjust the focus of an object image. The zooming lens (variator lens) is a component in a zoom lens system, and adjusts a magnification of the object image in directions to telephoto and wide-angle sides. The use of the voice coil motor in the lens driving apparatus is effective in reducing the size of the digital camera, video camera and the like, and reducing noise in driving the lens.
U.S. Pat. No. 5,939,804 (corresponding to JP-A 10-225083) discloses two yokes opposed to one another. The yokes have the magnet in combination with the coil for supporting the lens. A magnetic sensor is disposed between the yokes as a position detector. Those operate for reducing influence of leaked magnetic flux to the magnetic sensor. Also, JP-A 2011-123432 discloses the yokes to constitute two magnetic circuits for driving. There are four side lines of an image sensor. A first one of the yokes is disposed in parallel with a first one of the side lines of the image sensor in an optical axis direction. A second one of the yokes is disposed in parallel with a second one of the side lines of the image sensor in an optical axis direction, the second side line being adjacent to the first side line. A powering direction of the coil in a first one of the magnetic circuits is set opposite to that of the coil in a second one of the magnetic circuits with reference to the optical axis direction. Polarity of a first one of the magnets on the side of the image sensor is set opposite to that of a second of the magnets on the side of the image sensor. Thus, the image sensor is protected from magnetism.
It is technically known that flux density of the magnet is changeable with time or changes in ambient temperature. Even while a constant drive current flows through the coil, a value of the electro-magnetic force being generated may differ according to the ambient temperature or with time. Irregularity occurs in a moving speed of the lens driven by the electro-magnetic force upon occurrence of irregularity in the electro-magnetic force, so that the lens cannot be moved stably. The lens disclosed in U.S. Pat. No. 5,939,804 and JP-A 2011-123432 is the focus lens or the zooming lens (variator lens) in a lens driving method. Irregularity may occur in focusing or an adjustment speed for the magnification. Unfamiliar feeling in manipulation of a user is likely to occur.
While the coil is moved along the magnet in the optical axis direction to drive the lens, flux density of magnetism received from the magnet to the coil is characteristically smaller at end points of the magnet than at the center of the magnet in relation to the optical axis direction. In case a constant driving current flows through the coil, the electro-magnetic force generated by the magnet is smaller at its end points than at its center. Thus, the moving speed of the lens moving along the magnet with the coil becomes lower at the end points of the magnet than at its center. It may be possible to suppress changes in the moving speed of the lens by limiting a moving range of the coil to a main area of the magnet including the center. However, reduction of a size of the digital camera, video camera and the like has been focused in the industrial field. Assuming that the moving range of the coil is limited to the main area including the center of the magnet, areas at the end points are not utilized in a manner inconsistent to the idea of the size reduction. There is no known technique for stabilizing the moving speed of the lens in a structure where the moving range of the coil includes not only the center of the magnet but its end points.